1. Field of the Invention
The present invention relates to an optical recording medium and a method of producing the optical recording medium.
2. Discussion of Background
Recently overwrite type CDs (Compact Disks) have been actively developed. This is because the overwrite type CDs, unlike conventional CD, have the features that the user can record information therein as desired, and the recorded information can be reproduced therefrom by use of a commercially available CD player since the signals recorded in the overwrite type CDs satisfy the conventional CD standards for signals for recording.
An example of such an overwrite type CD is proposed in Japanese Laid-Open Patent Application 2-42652. This overwrite type CD comprises a substrate, a light absorbing layer which is provided by the spin coating of a dye on the substrate, and a metal reflection layer provided on the back side of the substrate opposite to the light absorbing light.
Another example of such an overwrite type CD is proposed in Japanese Laid-Open Patent Application 2-132656. This overwrite type CD comprises a light absorbing layer comprising a dye and is caused to satisfy the conventional CD standards for signals for recording by appropriately selecting the double refractive index and thickness of the light absorbing layer.
The overwrite type CD using dyes as disclosed in Japanese Laid-Open Patent Applications 2-42652 and 2-132656, however, do not have a sufficient light resistance for use in practice. To be more specific, these overwrite type CDs have the shortcoming that the signal characteristics thereof change, when exposed to sun light for an extended period of time, to such an extent that they cannot satisfy the conventional CD standards. This is because the properties of the dye materials used in the light absorbing layers thereof, in particular, conventional cyanine dyes, are changed when exposed to light.
In order to control such changes in the properties of the dye materials, it has been proposed to contain a light stabilizer in the light absorbing layer as disclosed in Japanese Laid-Open Patent Application 63-159090. However, when the content of the light stabilizer in the light absorbing layer is small, for instance, less than 20 wt. %, a sufficient light resistance for use in practice cannot be obtained, while when the content of the light stabilizer in the light absorbing light is large, for instance, 20 wt. % or more, the optical and/or thermal characteristics of the light absorbing layer are changed and the signal recording and reproduction characteristics of the light absorbing layer deteriorate in many respects.
As another means for improving the light resistance of the light absorbing layer, it is proposed as disclosed in Japanese Laid-Open Patent Applications 58-183296 and 58-37851 that a phthalocyanine compound, which is a dye with high light resistance, be employed as a material for the light absorbing layer. However, the solubility of the phthalocyanine compound in a solvent is too poor to form a film of the phthalocyanine compound by spin coating. In addition, it is difficult to obtain the optical characteristics required for the light absorbing layer, in particular, high refractive index, by use of such a phthalocyanine compound.
This is because the optical characteristics of the light absorbing layer depend upon the absorptivity coefficient of the layer (i.e. the absorbance per unit thickness of the layer) at a wavelength close to the wavelength of a laser beam used for recording and reproducing, so that there is required a sufficiently large absorptivity coefficient to satisfy the optical characteristics required for the overwrite type CD. However, a film layer of such a phthalocyanine compound has an absorptivity coefficient which is smaller than that of a film layer of the conventional cyanine dye. To be more specific, the absorptivity coefficient of the film layer of the phthalocyanine compound is about 1/3 to 1/2 that of the film layer of the cyanine dye.